What Causes Severe Pain In The Neck And The Back Of The Head?

My nine year old son has been complaining of his neck and back of head hurting since last night. He is playing off and on all day. This afternoon he fell asleep when he got up was running a temp of 102.2. I gave him tylenol. Fever has gone down but is still having pain in neck and head. He did tell me that he fell and hit his head on the ground yesterday. He also has poison ivy. Should I be concerned??